The disclosure of Japanese Patent Application No. 2000-149824 filed on May 17, 2000, including the specification, drawings and abstract, is incorporated herein by reference in its entirety.
1. Field of the Invention
The invention relates to an internal combustion engine having an exhaust gas recirculation passage through which exhaust gas discharged from a combustion chamber is recirculated into an intake passage of the engine, and also relates to a method for controlling such an internal combustion engine.
2. Description of Related Art
In one type of conventional internal combustion engines, an exhaust gas recirculation passage having an exhaust gas recirculation control valve is provided for recirculating exhaust gas discharged from a combustion chamber into an intake passage of the engine, and a catalyst for purifying recirculated exhaust gas that flows through the exhaust gas recirculation passage is disposed within the exhaust gas recirculation passage, while a throttle valve is disposed within the engine intake passage. In this type of internal combustion engine, as the amount of recirculated exhaust gas supplied into the combustion chamber increases, the amount of soot produced gradually increases and reaches a peak. As the amount of recirculated exhaust gas supplied into the combustion chamber further increases, the temperature of fuel and its surrounding gas at the time of combustion in the combustion chamber becomes lower than a temperature at which soot is normally produced, and therefore almost no soot is produced. Thus, the internal combustion engine is able to burn an air-fuel mixture almost without producing any soot, by controlling the amount of recirculated exhaust gas supplied into the combustion chamber to be greater than that with which the amount of soot produced reaches a peak. An example of this type of internal combustion engine is disclosed in, for example, Japanese Laid-open Patent Publication No. 2000-8964.
When the engine operates in the above-described combustion mode in which the amount of recirculated exhaust gas supplied into the combustion chamber is greater than that with which the amount of soot produced reaches a peak, and thus almost no soot is produced, and the air-fuel ratio is slightly lean, the opening amount of an exhaust gas recirculation control valve is reduced and the opening amount of the throttle valve is also reduced, as compared with the case where the air-fuel ratio is lean, as disclosed in the above-identified publication JP-A-2000-8964. However, there is no disclosure in this publication as to how the opening amounts of the exhaust gas recirculation control valve and the throttle valve are controlled when the air-fuel ratio is shifted to the rich side of the stoichiometric ratio during an engine operation in which the amount of recirculated exhaust gas supplied into the combustion chamber is greater than that with which the amount of soot produced reaches a peak and almost no soot is produced. If the opening amount of the exhaust gas recirculation control valve is not reduced while the opening amount of the throttle valve is reduced when the air-fuel ratio is shifted to the rich side of the stoichiometric ratio during the engine operation as described above, the amount of recirculation exhaust gas increases. As a result, the combustion temperature decreases, and the temperature of recirculated exhaust gas is accordingly lowered, resulting in a reduction in the temperature of the catalyst for purifying the recirculated exhaust gas. Consequently, the catalyst for purifying the recirculated exhaust gas gets poisoned by SOF (soluble organic fraction), namely, the catalyst suffers from so-called SOF poisoning.
It is an object of the invention to provide an internal combustion engine capable of substantially preventing a catalyst for purifying recirculated exhaust gas from suffering SOF poisoning, when the air-fuel ratio is shifted to a rich side of the stoichiometric ratio during an engine operation in which the amount of recirculated exhaust gas supplied into a combustion chamber is greater than that with which the amount of soot produced reaches a peak and substantially no soot is produced. It is another object of the invention to provide a method for controlling such an internal combustion engine.
To accomplish the above and/or other objects, one aspect of the invention provides an internal combustion engine, which includes (1) an exhaust gas recirculation control valve disposed in an exhaust gas recirculation passage through which an exhaust gas discharged from a combustion chamber is recirculated into an intake passage of the engine, (2) a catalyst disposed in the exhaust gas recirculation passage, the catalyst being operable to purify recirculated exhaust gas passing through the exhaust gas recirculation passage, and (3) a throttle valve disposed in the intake passage. In the internal combustion engine, an amount of soot produced gradually increases and reaches a peak as an amount of the recirculated exhaust gas supplied to the combustion chamber increases, and, when the amount of the recirculated exhaust gas supplied to the combustion chamber further increases, a temperature of fuel and surrounding gas at the time of combustion in the combustion chamber becomes lower than temperatures that permit production of soot. Thus, the internal combustion engine may operate in a combustion mode in which the amount of the recirculated exhaust gas supplied to the combustion chamber is larger than that of the recirculated exhaust gas with which the amount of soot produced reaches a peak, and almost no soot is produced. A controller of the engine according to the invention reduces an opening amount of the exhaust gas recirculation control valve and reduces an opening amount of the throttle valve when the air-fuel ratio is shifted to a rich side of the stoichiometric ratio while the engine is operating in the combustion mode in which almost no soot is produced, as compared with when the air-fuel ratio is not shifted to the rich side of the stoichiometric ratio.
In the internal combustion engine according to one aspect of the invention as described above, the opening amount of the exhaust gas recirculation control valve is reduced and the opening amount of the throttle valve is also reduced when the air-fuel ratio is shifted to a rich side of the stoichiometric ratio while the engine is operating in the low-temperature combustion mode in which almost no soot is produced, as compared with the case where the air-fuel ratio is not shifted to the rich side of the stoichiometric ratio. By reducing the opening amount of the throttle valve, the pumping loss is increased so that the exhaust gas temperature can be raised. Furthermore, the opening amount of the exhaust gas recirculation control valve is reduced so as to prevent the amount of the recirculated exhaust gas from increasing with a reduction in the opening amount of the throttle valve. This makes it possible to suppress reduction in the combustion temperature and also suppress reduction in the recirculated exhaust gas temperature. Consequently, an otherwise possible reduction in the temperature of the catalyst for purifying the recirculated exhaust gas is prevented, and SOF poisoning of this catalyst can be advantageously suppressed or avoided.
In one preferred form of the invention, an amount of reduction in the opening amount of the exhaust gas recirculation valve is increased with a reduction in a load of the internal combustion engine.
Since the combustion temperature tends to be lowered as the load becomes lighter, the amount of reduction in the opening amount of the exhaust gas recirculation control valve is increased as the load decreases. This arrangement makes it possible to prevent the EGR gas purifying catalyst from suffering from SOF poisoning due to an otherwise possible reduction in the combustion temperature with a reduction in the load.
In another preferred form of the invention, an amount of reduction in the opening amount of the exhaust gas recirculation valve is increased with an increase in a revolution speed of the internal combustion engine.
Since the intake pipe negative pressure increases and the amount of the recirculated exhaust gas tends to increase as the engine speed becomes higher, the amount of reduction in the opening amount of the exhaust gas recirculation control valve is increased as the engine speed increases. This arrangement makes it possible to prevent the EGR gas purifying catalyst from suffering from SOF poisoning due to an otherwise possible increase in the amount of the recirculated exhaust gas resulting from an increase in the engine speed.
In a further preferred form of the invention, an amount of reduction in the opening amount of the exhaust gas recirculation valve is increased as the air-fuel ratio is shifted to a richer side of stoichiometric ratio.
Since the combustion temperature tends to decrease as the air-fuel ratio is shifted further to the rich side from the stoichiometric ratio, the amount of reduction in the opening amount of the exhaust gas recirculation control valve is increased as the air-fuel ratio is shifted further to the richer side. This arrangement makes it possible to prevent the EGR gas purifying catalyst from suffering from SOF poisoning due to an otherwise possible reduction in the combustion temperature resulting from a shift of the air-fuel ratio to the richer side.